The invention relates to a vane actuator for use in controlling movement of a plurality of guide or stator vanes in a gas turbine engine.
An axial flow, multi stage compressor for a gas turbine engine typically includes alternate rows of rotating (rotor) blades and stationary (stator) or guide vanes to accelerate and diffuse the flow of air to the turbine. As each stage of a multi stage compressor has air flow characteristics that are different from those of the preceding and subsequent stages, it is necessary to ensure the characteristics of each stage are carefully matched. In order to achieve reasonable matching over a range of operating conditions, the vanes are actuable to direct the air flow onto the subsequent rotor vanes at an acceptable angle.
A typical drive mechanism for the variable guide or stator vanes of a multi stage compressor is shown in FIG. 1. The vanes (not shown) are coupled to a retaining or carrier ring 10 (also known as a unison ring) which is driven, through a suitable linkage, by means of an actuator arrangement comprising a first, master actuator 12 and a second, slave actuator 14.
Each of the first and second actuators 12, 14 includes an electrohydraulic servo valve which is arranged to control the flow of pressurised fluid to respective first and second chambers of a linear actuator piston coupled to the carrier ring 10. The servo valve is supplied with an electrical current which energises a winding of the servo valve to control the position of a spool valve and, hence, the flow of fluid to the first and second chambers. By controlling the position of the servo valve, the pressure of fluid within the first and second chambers can be varied so as to drive the carrier ring 10, and hence the guide vanes, into the required position. For most of their service life, the hydraulic actuation system serves to retain the variable vanes in a fixed position appropriate for engine cruising speed. During take off and landing, the hydraulic actuation system is operated to adjust the position of the guide vanes to compensate for variations in airflow through the compressor.
One problem associated with such hydraulic actuation systems is that, in the event of loss of control of the system or pressure within the first and second control chambers, the guide vanes become free to move in the compressor airflow. In order to avoid the possibility of engine overspeed in the event of such a failure, the engine will be shut down.
It is an object of the present invention to provide an actuator suitable for use in moving variable vanes of a gas turbine engine which removes or alleviates the aforementioned problem.
According to the present invention, there is provided a vane actuator for use in a turbine engine, comprising a carrier member carrying a plurality of vanes which is angularly movable, in use, so as to vary the position of the vanes relative to an airflow through the engine, an electrical drive arrangement comprising an input shaft which is arranged to drive an output shaft coupled to the carrier member, the electrical drive arrangement comprising a brake arrangement arranged to apply a braking load to the input shaft in the event that an interruption occurs in the electrical drive arrangement.
The invention provides the advantage that, in the event of the occurrence of a fault in the electrical drive arrangement, for example due to an electrical supply failure, a power-off brake applies a braking force to the input shaft.
The application of the braking force to the input shaft serves to lock the carrier member, and hence the vanes, in a fixed position relative to the airflow. Undesirable movement of the vanes is therefore substantially avoided. Thus, in the event of an electrical supply failure, there is no risk of surge or need to take the precaution of immediately shutting down the engine in anticipation of engine overspeed.
A further advantage of the present invention is that the use of electrically driven actuation systems on aircraft offers the potential for increased aircraft reliability and efficiency and reduced weight, maintenance and manufacturing cost.
Preferably, the output shaft has an input, drive end which is coupled to the input shaft through a gear arrangement and an output, driven end which is coupled to the carrier member. The carrier member preferably takes the form of a carrier ring.
In a preferred embodiment, the electrical drive arrangement comprises a motor, the input shaft being rotatable under the influence of the motor.
The brake arrangement preferably comprises a plurality of first brake elements which are rotatable with the input shaft and which are engageable with respective ones of a plurality of second brake elements to control the braking load applied to the input shaft.
Preferably, the brake arrangement further comprises an electromagnetic actuator comprising an armature which is carried by the input shaft and which is movable under the influence of a magnetic field generated by an energisable winding.
The electromagnetic actuator may be arranged such that energisation of the winding causes the armature to be attracted towards the winding, thereby causing the first and second brake elements to disengage from one another to remove the braking load from the input shaft, de-energisation of the winding causing the first and second brake elements to move into engagement with one another under the action of a return spring such that the braking load is applied to the input shaft.
The vane actuator may include a ballscrew actuator comprising an input member which is angularly movable upon rotation of the input shaft, the output shaft being axially movable upon angular movement of the input member. The output shaft may be coupled to a linkage, the linkage being arranged to impart angular movement to the carrier ring upon axial movement of the output shaft.
The input member may be provided with a screw thread formation including a helical groove, spherical elements being carried by the output shaft and being in rolling engagement in said helical groove to form a ballscrew coupling between the input member and the output shaft.
The input member of tie ballscrew actuator may be provided with a flange to which the input shaft is coupled through a gear arrangement. Preferably, the ballscrew actuator comprises overload protection means for applying a braking force to the input member in the event that an axial overload is applied to the output shaft, thereby to prevent loading of the electrical drive arrangement.
For example, the actuator may be provided with first and second abutment surfaces, a region of the input member, for example a flange, being engageable with one or the other of the first or second abutment surfaces in the event that the overload is applied to the output shaft, frictional engagement between the region of the input member and the first or second abutment surface causing the braking load to be applied to the input member to arrest rotation thereof.
As rotation of the input member is prevented upon engagement between the flange and the first or second abutment surface, any loading of the electrical drive or gear arrangement, which may otherwise cause substantial position change of the vanes, is limited to an acceptable level.
In an alternative embodiment, the actuator may include a rotary actuator.
In a preferred embodiment, the actuator includes first and second ballscrew actuators having respective output shafts, each of the output shafts being coupled to the carrier member and being coupled together through a common drive and synchronisation shaft to ensure axial movement of the output shafts is substantially synchronised.
The occurrence of a fault condition in the electrical drive arrangement may originate either within the electrical drive arrangement itself or may be generated externally to the electrical drive arrangement, and typically may arise as a result of either a total or partial electrical supply failure.
Electrical supply may be intentionally interrupted during fixed engine operating conditions.